Process and apparatus for digesting fibrous material



Dec; 17,' 1940. A. D. MERRILL PROCESS AND APPARATUS FOR DIGESTING FIBROUS MATERIAL Filed June 26, 1935 QEESSuu Qsw uh QEWIQY 2 Patented Dec.1 7,1940

PROCESS APPABATU S FOR DIGE STING rmaons MATERIAL Albert n. MerrilLWatertown, N. Y. Application June 26, 1935', Serial No. 28,578

(or. ea-r) Claims.

This invention relates to an improved process of and apparatus for digesting fibrous material, and more particularly to a method of controlling the characteristics of the digestion liquor during the treating period.

- As is known, in the preparation of sulphite pulp, the ultimate reaction involves an interaction between the lignin or its derivatives and the liquor. It is probable, or at least generally assumed, that the bisulphite chemically combines with the lignin to form ligno-sulphonic derivatives, the action being generally conceived as a hydrolytic scission, of the cellulose-lignin aggregates or esters. Coincident-ally a number of more or less complicated secondary hydrolytic reactions occur, resulting in the formation of such products as sugars, variant forms of hydrolyzed cellulose and the like. Usually a base, such as calcium or magnesium, is employed upon the theory that these combine with certain products of hydrolysis and prevent further reaction of such products. It is generally assumed that the reactions, inter alia, include the formation of organic acids which then combine with the sulphites to displace and set free an equivalent amount of sulphurous acid, this liberation being greatest in the latter part of the cook and necessitating a blow down to diminish the pressure.

Whatever may be the precise mechanism of the reaction, it'is a practical fact that the concentration of the sulphurous acid'is the most important factor, considered not only as a component of the reagent, i. e. in chemical combination with the base, but also in the free or uncombined state.

This importance has of course been recognized and many suggestions have .been advanced as to methods of maintaining the acid concentration during the reactions. Of these perhaps the one of most practical importance and value is the recirculation of the free gas back into the body of the digestion liquor. The usual method of accomplishing this is to couple agas line and a liquor line into the same circuit and to withdraw and-readmit both gas and liquor by means of a Ramiflcations of such a method include ma-- the; suggestion of effectively condensing and/or -"..fahsor bin'g the gas during this passage, as by 50 means'of a jet condenser or eductor.

The. present invention relates to this general of treatment, that is to a method of maintainln'g acid concentrations by returning free gas either continuouslyor intermittently to the body of the'liqu'o'r in the digestion zone. In common handling or manipulating a digestion liquor with prior art suggestions also the present invention utilizes a recirculating stream of liquor as the carrier and absorber of such gas. The present method however is'fundamentally different from those heretofore employed principally in that it involves the selective recirculation of gas and liquor. By reason of this the chemical characteristics of the cooking liquor may be accurately controlled and, if desired, optionally varied during the progress of the cook to thus obtain an improved result. Other concomitant advantages obtain which will be described hereinafter or which will clearly appear from a consideration of the novel method. j

An object of the present invention therefore is to provide an improved process of digesting fibrous materials.

Another object is to provide a new method of which includes as one of its components a relatively volatile gas.

Another object is to provide animproved type of gas-liquor circuit for use in the digestion of fibrous materials.

A further object is to provide a'novel method of controlling the chemical characteristics of a digestion liquor during the treatment of fibrous materials.

With these and other equally important objects in' view, the invention comprehends the concept of digesting fibrous material with a digestion liquor, one component of which is a volatile material, such as sulphur dioxide, and selectively recycling the liquor and gaseous component during the treatment.

In order to enable a more ready comprehension of the underlying principles of the present invention, a typical arrangement of apparatus by which it may be effectuated is shown in the 4 single figure of the accompanying drawing, the purpose being to illustrate the fundamental mode of operation rather than to exclusively define the apparatus which is available. For this reason and further to simplify the, drawing, conventional units have been selected.

As indicated hereinbefore, it has been suggested prior to this time to withdraw evolved gases from the upper portion of a digester and introduce them into a recirculating stream-of digesterliquor. In such prior methods the gas and liquor lines were connected" in the same circuit, usually through an educt'or, and the amount of gas which was withdrawn in any given time was d pend ntupon or a function of the velocity of control of the amount of recycled gas as compared to the amount of recycled liquor. In this way, therefore, control of the concentration of the acid by means of optionally varying the partial pressure of the sulphur dioxide gas in the vapor space may be obtained without necessarily recycling an equivalent quantity of liquor. Thus, utilizing the principles of the present invention, a substantially constant fiow of liquor may be established in the recycle circuit, and the flow of gas may be varied independently of the liquor to adjust the concentration at any desired stage of the cook. Furthermore, in the improved embodiment of the invention the gas recycle line, due to this fact of independent control, may be utilized for other purposes, such for example as rapidly relieving gases during the latter stage of the cook or for withdrawing excess gases from the digestion zone for transportation to some other absorption system. These and other advantages may be achieved in operating a process which may be carried out in the apparatus shown.

In the drawing I indicates an acid storage tank of any suitable size and construction. This, as is known, may be supplied with fresh acid from the acid towers by means of the supply line 2 and pump 3. Liquor may be taken from storage tank by the pump '4 and forced through line 5, check valve 6 and eductor to the drop leg 8. This drop leg is connected at its lower end with the second eductor 9 and thence through the branch I!) to the accumulator The eductors are provided with control valves 1' and 9 respectively, the purpose and operation of which is described in prior Patent 1,888,632.

With such an assemblage it will be appreciated that relatively cool acid may be pumped from the acid storage through eductor 1, drop leg 8 and eductor 9-into the accumulator and that during such passage acid may be employed to absorb gas and condense condensible vapors.

As described more fully in Patent 1,918,459, the absorption or condensation of gases and vapors may be accentuated by associating the eductor 9 with an accumulator recirculation circuit. As shown; this may comprise the inlet branch l2, controlled by valve l2, pump l3 and branch line 14, which latter communicates with the liquid inlet of eductor 9. With this type of circuit liquor may be withdrawn and forced by the pump l3 at any desired velocity through branch l4 and into eductor 9, and thence to the accumulator through the branch l0. Liquor flowing through line H absorbs and condenses gas and/or vapors flowing downwardly through the dro leg 8 in admixture with liquor.

The emperature of the acid within the accumulator may be more accurately controlled by interposing, if desired, a heater in this recircu- 'lation line, as fully described in Patent 1,918,459.

The pressure within the accumulator may likewise .be controlled. As shown, a vent line 5 leads from the upper or vapor section of the spherical accumulator andis immersed to a considerable depth in the liquor contained in acid v or recording instruments (not shown).

one or more digesters l8. These may be of typical or conventional construction and include the removable manhole cover I! and a valve blow of! elbow. Each one ofthe digesters is provided with a gauge glass It, for the purpose of determining the liquor level, and pressure indicating The reliei' line 28, having the interposed control valve 20, connects with the main or high pressure header 2|. Similarly the side relief 22, having control valve 22', communicates with header 2|. This latter, as shown, leads to the eductor I. Fluids may flow from the digester through either or bothv lines 20 and 22 into header 2| and thence in the manner described through the drop leg 8 into the accumulator. Such hot fluids precondition and preheat the cooler acid flowing through line 5.

The digester is also provided with a low pressure relief line 23, having a control valve 23', and

, is connected to the header 24. This latter header,

which carries the low pressure gases, terminates below the liquid level in the acid storage. The digesters are also provided with the air vent branches 25, controlled by valve 25', through which displaced air may be vented during filling of the digester, in a manner to be particularly described.

The lower-end of the digester is connected, by suitable branch lines 26, to a steam header (not shown) in the usual manner. A bypass line 2'! rectly cooking the material in the digester, may

be fed from the accumulator through line 30, controlled by valve 30', and pump 32, into the feed header 33. From this header the hot preconditioned acid may be admitted to the several 'digesters through the branches 34, controlled by valves 34'.

By opening valve 30 and valve 34' on a particular digester, hot acid may be forced by means of the pump (supplemented by the existing pressure in the accumulator) into the bottom of the digester until such digester is filled under any desired hydrostatic pressure. After filling the digester may be sealed off from the supply in accumulator II by manipulating valve 34'.

In a process of the present type it is desirable to recirculate both gas and liquor to and from the digester. For this purpose the recirculation circuits of the type shown are employed. As will be observed from an inspection of the draw-. ing, liquor may be withdrawn from the upper portion of the digester through the line 40 by means of the pump 4i andthen forced through line 42, heat control unit 43 and eductor (to the digester. If desired the eductor 44 may be upper and returning it to the lower portion of the digester, it will readily be appreciated that by the proper placement of the lines or by the use of proper branch and bypassconnections, the circulation may be in the opposite sense, namely from the bottom to the upper portion. It is further to be understood that the invention comprehends the utilization of these two circulations,

namely top to bottom and bottom to top, during any given treatment. Since the method of achieving this is well known to those skilled in the art and since a showing would unduly complicate the drawing, such reverse circuits are not shown.

Consonant with the principles of the invention as described hereinbefore, means are provided to withdraw gas from the upper or gas space of the digester and force it by independent means into the bottom of the digester. Preferably, since a liquid recycle circuit is being employed, the two are combined in such a way as to absorb the gases and condense vapors passing through the gas line, and for this purpose the eductor 44 is utilized.

Such separate gas recycle circuit may comprise the line 50, one end of which is positioned within the vapor space of the digester. The end of this line is suitably screened in the manner well known in the art. The other end of the line 50 is connected to the intake of a compressor or blower 5!, such as a Roots blower or a Nash blower. The outlet of the blower is connected through line 52 which may discharge directly, as shown, into the digester. Connected to the line 52 are a number of branch lines through which the gas may be caused to flow. Thus gas withdrawn from the upper portion of the digester may be forced through branch 53, which branch is controlled by valve 53'. This branch connects with the discharge or high pressure side of pump 4| and such points of connection may be made either anterior or posterior to the heater 43.

If desired the discharge of the pump and branch 53 may connect with an enlarged chamber to provide a greater area for the mixing of the gas and liquid. To facilitate such mixing agitating means may .be provided within the container. In any event it will be observed that in such circumstances gas and liquid may separately and independently be withdrawn from the upper portion of the digester and. admixed. and combined in a zone of high hydrostatic pressure, thus insuring a greater absorption of gas and condensation of c'onde'nsible components. 'Either before or after such admixture the fluids may be passed through the heater so as to raise the temperature toany desired value.

Likewise if desired, the branch 53 may be cut out by closing the valve 53' and the gas forced through line 52 and branch 54 into eductor 44. The flowof the gas through this branch may be controlled by valve 54'. Wherever the circumstances require both branches 53 and 54 may be cut out and valve 55' opened so as to pass the gas through lines 52 and 55 into the bottom of the digester.

Similarly it will be understood that if and when desired valve 55' may be closed and both valves 53'. and 54' opened so that gases from line 52 flow into thebranch 42 and into the liquid line through the eductor. When either line 53 by opening the or 54 is opened and operating it will be appreciated that the suction created in these lines supplements the action of the blower 5i in rapidly flowing the gas through the recycle, line.

As noted herei'nbe'fore theseparate gas recycle line may be employed for additional purposes during the course of treatment. Thus valves 53',

54' and 55' may be closed and gases may be withdrawn from the upper portion of the digester and caused to flow through the valve branch line 56 for suitable disposition. Thus for example line 56 may be connected to the accumulator II or by suitable conduits to another digester for the purpose of increasing the free acid' content and adding heat units to the .liquor in such digester. Again a branch from line 56 may lead to a'low pressure accumulator or storage tank' so that gases vented during the filling of the digester may be absorbed in such tank. By utilizing a blower in this line the final blown down period can be materially shortened inasmuch as the pressure can be rapidly relieved by the blower rather than by the slower method employed heretofore. 1

Also it will be observed that the separate gas line may be used-together with the liquid recycle line at or near the end of the cook for the purpose of degassing the liquor. liquor may be withdrawn from the bottom portion of the digester, in a manner heretofore described, and pumped to the top of the digester to be sprayed therein. By operating the blower 51 at this time the pressure in the vapor space may be materially diminished and the free gas quickly evolved from the liquor and withdrawn through lines 52 and 56 for utilization elsewhere in the system. It is to be observed that in these circumstances degassing is very rapid because the blower 5| tends to establish a reduced pressure in the vapor space of the digester and thus accelerate the evolution of gas from the-recycle liquor which is sprayed into the top. Such method therefore insures not only economies in operating time but maximum recovery of chemical values.

It will be appreciated that inasmuch as the centrifugal pump 4i and the blower or equivalent motive power 5i are independently operated, the

new of the respective materials, namely the gas.

In these circumstances and liquor, may be varied to any desired degree and at any time. In this manner; therefore, it

is possible to insure a'rapid recirculation of gas,

and a. relatively slower recirculation of liquid.

A This is .in sharp, .contradistinction to prior art proposals, in which the flow of gas was of necessity a function of the flow ofrecycled liquor. This type of procedure therefore establishes amethod of controlling the partial pressure of the gas in the vapor space which could-not be achieved heretoforeu The degree of this control can of course be modifiedover a relatively wide range. Thus, if desired, there may be connected into the gas recycle line aseparate source of gas, that is to'say gas from an extraneous source may be admitted to the gas recycle line and thuscaused to enter the digester circuit to increase .the partial pressure of the S02 in the digestion zone. This may readily be done by connecting to the gas recycle line a tank of gaseous or liquid S02. As noted hereinbefore,

branch line 56 to the desired degree gas may be withdrawn from the circuit to any desired extent and, as explained, towards the end of the process may be used, in conjunction with the liquid recycle line, to substantially completely evacuate the circuit of free gas.

In addition to these major advantages by reamum concentration and temperature.

son of which the chemical characteristics of the process may be controlled, it will be observed that the particular arrangements provide so-to-speak mechanical advantages. For example, by utilizing the separate blower forthe gas and the centrifugal pump for the liquid, the fluids may be combined as described in a high pressure zone, on the discharge side of pump 4|, thus insuring the same effective operation of both the pump and the blower and an optimum absorption of gas. By utilizing such an arrangement and by placingthe liquor inlet 40 well below the normal liquid level in the digester, it will be appreciated thatall difliculties of gas binding of the centrifugal pump are avoided.

The complete process for producing pulp from ligno-cellulosic material will have been appreciated from the foregoing description. In carrying out such a process the raw material, such as wood chips, is admitted to the digester through the open manhole l1. After the digesterhas been completely filled with chips the cover is bolted on and valve 25' is opened, valves 20', 22' and 23' at this. time being closed. Hot preconditioned liquor is then forced from accumulator H, in the manner described, through branch 34, into the bottom of the digester. As the liquid level rises air is continuously displaced and is vented throughthe open valve 25'. When the air is completely vented and S02 gas begins to discharge at valve 25 the valve in line 50 may be opened and blower 5| operat d to withdraw gases from the digester and pass th hrough line 56 to a branch line (not shown) into the acid storage tank., When such gases are withdrawn and the digester filled, the valve in line 50 may be closed and operation of the blower 5| discontinued. I'hen preferably the pump 30 is continued in operation until a high hydrostatic pressure is built up upon the liquid. This pressure (gauge) may, for example, vary from 40 to 300 lbs. or more per sq. in. The chips are then in contact with liquid under high pressure and at an elevated temperature but below reaction temperatures, the temperature of the acid withdrawn from the accumulator l I being normally at about 70 to C.

After the high hydrostatic pressure has been built up on the acid, valve 34' is closed and the liquid recirculated to and from the digester for a period of from one to six hours, so as to insure a thorough saturation of the chips with the liquor. During the saturation period no recirculation of gas is necessary. At this time however, if desired, gas may be admitted to the liquid recirculation line from an extraneous source so as to build up the partial pressure of the SO: in the digestion zone and thus increase the concentration of the acid during the saturation period without increasing the temperature. During the saturating period the temperature of the acid may be maintained at the optimum value by indirectly heating recycle liquid in the heater 43. In this manner the chips may be thoroughly saturated with hot acid at an opti- If the optimum saturating concentration is less.than the concentration to be employed for cooking, suflicient gas can be withdrawn from the circuit to reduce the concentration to the desired value.

At the termination of the saturating period, which as noted above may vary over a considerable period of time, conditions within the digester are adjusted so as to bring the mass up to reaction temperatures. As shown in the drawing, this may be done by opening valve 22' and relieving liquid from the digester to lower the liquid level to the desired point. At this time also, if desired, the blower II may be operated for a short period of time to withdraw gas from the zone and thus to adjust the concentration of free gas prior to cooking. Then the mass is brought to reaction temperature by applying heat, either by means of a closed steam coil or preferably as shown by introducing direct steam through the branches 26.

Sometime after the initial admission of steam the valve 20' may be opened, thus venting fluids to the desired extent, from the digester to the header 2| and thence through the described circuit to the accumulator. I

After a period of time, which may vary depending upon the particular conditions of. the treatment, the valve 20' may be closed and the valve 22 in the side relief line opened to withdraw liquid and to maintain the liquid level at well known to those skilled in the art. by roper manipulation of the relief valves.

Either continuously or intermittently during the cooking period liquor may be withdrawn from the upper portion of the digester, through line 40, and forced under the pressure of pump ll into the lower portion. contemporaneously gas may be withdrawn from the vapor space of the digester through line 50, under the action of blower 5|, and then admixed with the liquid in a zone of high hydrostatic pressure, through line 53, or in the eductor through line 54, or finally whenever circumstances recommend, directly into the digester through line 55.

While the pump 4| has been shown asa centrifugal pump adapted, due to its operation, to force a steady stream of liquor through the circuit, it will be appreciated that this may be replaced with a reciprocating pump so as to introduce the liquor into the digester in a series of pulsating or surging streams, in the manner described for example in the prior application of T. L. Dunbar, Serial No. 636,312, filed October 5, 1932. This typeof introduction of liquor provides the advantage of agitating or loosening up the body of the pulp. As noted above, during the cooking period liquor may be withdrawn from the bottom and admitted to the'top of the di-- gester and a combination 01' these two types of recirculation, namely from top to bottom and bottom to top, maybe utilized.

With this type of operation-the quantity of gas recycled at any particular time may be varied entirely independently of the quantity of liquor, and the gas and liquor may be combined under such conditions as to insure optimum absorption and heat control. By utilizing the gas recycle line, as noted hereinbefore, SO: gas (or its equivalent) may be introduced from an extraneous source to build up the partial pressure of the gas in the digestion zone, or any predetermined amount 01' gas may be bled from this line.

It will thus be appreciated that the concentration of the acid during any stage in the cook may quite accurately be controlled without modifying to any considerable degree the original concentration of the acid, that is to say the concentra-' tion of the acid at the start of the cook.

As the reactions involved in the digestion of 1s ligno-cellulose proceed, that is to say in the latter part of the cook, organic acids formed combine with sulphites present and tend to displace an equivalent amount of sulphurous acid. In order to maintain the proper pressure conditions the evolved gas should, as is known by those skilled in the art, be withdrawn. In the present process this may be done as in the past by bleeding off through line 23 or by withdrawing such gas from the gas recycle line through branch 56, or by a secured, for the outlet branch 56 may be connected to an absorption system in which a higher pressure obtains than in the acid storage tank.

After the termination of the cook and before the pulp is discharged from the digester, as indicated hereinbefore, maximum recovery of chemical values may be obtained by utilizing the dual recycle circuit. For this purpose the liquid in the digester, now under relatively low pressure, may be withdrawn from the bottom and forced to and sprayed into the top of the digester. At this time the blower may be operated to set up a diminished pressure in the upper portion of the digester, thus increasing the evolution of S02 gas and withdrawing this from the digestion zone. Thus the residual or spent liquor may be denuded practically completely of its free gas by an improved type of operation which in fact comprises a rapid reduced pressure distillation.

If desired the liquid recycle line may be employed, by connecting it to suitable separate inlet lines, to force a washing solution through the pulp prior to discharge of the pulp to the pulp storage. After such discharge the pulp may be further processed in the manner well known to those in the art.

It will be observed, therefore, that the described system presents many advantages over those heretofore employed. The provision of a separately controlled gas recycle line enables a more accurate adjustment of pressure during any stage of the cook; it insures increased absorption of the free gas by enabling the utilization of a high pressure zone for the absorption, while at the same time it permits more efiicient operation of the liquid pump. The use or this type of gas recirculation system, as pointed out, permits the possibility of employing it"ror a number of other and advantageous purposes or functions, such for exsample as rapid degassing of the residual liquor in'the manner described.' While, therefore, the novel gas-liquid recycle circuit has been described with respect to employment on a digester and during the cooking period, it will be appreciated that the fundamental principles may be embodied in any step of the treatment in the. plant.

Therefore, while preferred procedures and apparatus have been described, it will be understood that these are given as illustrative of the underlying salient principles of the invention and not as delineating the scope of the invention.

I claim:v

1. In a process of digesting fibrous material, pumping acid liquor directly to and from a digester through an acid liquor circuit, separately pumping a stream of hot gas from the upper por- 'tion of the digester, and in an amount independent of the amount of liquor passing through the liquor circuit, admixing this with the stream of liquor in the high pressure section of the liquor circuit and introducing the admixed stream to the digester.

2. In a process of digesting fibrous material, pumping a stream of acid liquor through a circuit from one section to another section of the digester, during the cooking period, separately pumping a stream of gas from the upper portion of the digester and forcing it into the lower portion of the digester during the cooking period;

- then during the latter stages of the treatment withdrawing liquor from the bottom and spraying it into the top of the digester, and utilizing said gas circuit to impose a reduced pressure onthe digester to increase the evolution of gas from the liquor and to withdraw evolved gas from the digester.

3. An apparatus for digesting fibrous material comprising a digester, a liquor recycle line extending from the upper to the lower portion of the digester, a pump and heater and eductor in such line, a gas recycle line extending from the upper to the lower portion of the digester, a blower in such line, and means optionally to connect such gas line directly to such liquor line on the pressure side of the pump or through said eductor.

4. An apparatus for digesting fibrous material comprising a digester, a liquid recycle line extending from the upper to the lower portion of the digester, a pump and heater and eductor in such line, a gas recycle line extending from the upper to the lower portion of the digester, a blower in such line, and means to connect such gas line to such liquor line on the pressure side of the pump.

5. In a process of digesting fibrous material, filling a digester with the material, admitting acid to the digester and simultaneously displacing air therefrom, continuing the admission of acid and withdrawing evolved gas through a line having an interposed blower and conducting such gases to a storage tank; subsequently digesting the material at superatmospheric pressure with the liquor while recycling liquor to and from the digester and separately recycling gas to and from the digester through a circuit including the blower and at a rate of flow independent of the rate of flow of liquor through the liquor circuit.

- ALBERT D. MERRILL. 

